Polymeric materials have been widely used in devices for transportation, sports and recreation, construction, coatings, and other pursuits where the materials are in an environment where they may experience mechanical, chemical, radiative, thermal and other stress. These stresses lead to damage that range from large wounds to micro-cracks that can be difficult to detect and not readily reparable. Not only can the aesthetic qualities of the material be compromised by this damage, but the function lifetime of the device can be diminished. This damage need not be at the exposed surfaces of the device or in the bulk or continuous phase of a polymeric material, but can exist at an interface of a second material, such as a supporting surface or a reinforcing filler phase of a composite.
To this end, materials that can recover mechanical properties are advantageous. A self-healing polymer (SHP) has the potential to repair a wound and prevent the propagation of cracks or other wounds at the micro scale. To achieve this self-healing, two approaches have been pursued. In the first approach, self-healing results from the incorporation of micro-encapsulated uncured resin as a homogeneously distributed filler phase. Fracture of the material is intended to cause rupture of the micro-capsules with the release of resin whose polymerization repairs the fracture. A catalyst that is in the polymer phase, but is immobile or impermeable to the capsules, is also included in many formulations of these micro-capsule filled polymeric materials. In the second approach, reversible bonds are included in the self-healing polymeric material. The reversible bonds allow local remodeling of the damaged material.
Self-healing reversible bonds have most frequently been formed by the Diels-Alder (DA) cycloaddition reaction, which is a thermo-reversible reaction. DA reactions do not require additional chemicals, such as catalysts. The DA reaction is a concerted reaction between a four-π-electron system, for example, a 1,3-diene, with a 2-n-electron system, a dieneophile, for example, an alkene. There are numerous examples in the literature of the preparation of self-healing polymers where the diene or dieneophile is situated on the terminal ends of a polymer, as pendent groups on a polymer chain, or are used as complementary functionality for the formation of a step-growth polymer from a bis-diene monomer and a complementary bis-dieneophile monomer or from an asymmetric monomer having a diene at one position and a dieneophile at another position of the monomer. Being a concerted reaction, there is no intermediate, for example, a radical or ionic species, in the DA reaction that can cause unwanted side-reactions.
The most frequently employed diene and dieneophile pair is that of furan and maleimide, respectively. The diene of the cyclic furan moiety is frozen in an s-cis conformation within the ring. The s-cis conformation is required for the concerted reaction. The maleimide provides a very reactive dieneophile due to the electron withdrawal carbonyl groups thereon. Polymers made in this fashion typically do not have a high degree of polymerization (DP) and the nature of the polymerization process affects the DP obtained. Typically, polymerization is carried out at temperatures in excess of 90° C. The step-growth polymerization using DA cycloaddition reactions has been performed with multi-maleimide monomers with multi-furanyl monomers. The resulting networks are highly cross-linked and display self-healing properties. Fracture of these polymers often occur where the imposed bond breaking occurs by the retro-Diels-Alder (RDA) reaction rather than cleavage of other covalent bonds, as the enthalpy of the RDA reaction is approximately 96 kJ/mol, as opposed to a normal C—C bond energy of approximately 350 kJ/mol. When the polymer or network is wounded, typically there is a separation of the fractured surfaces that requires a mechanical forcing of the fractured surfaces to be placed into intimate contact.
Shape-memory polymers (SMPs), more specifically thermo-responsive shape memory polymers, are relatively lightly cross-linked networks where the shape of the original cross-linked network can be modified when the polymer is deformed at a temperature above its glass-transition temperature (Tg) and the deformed shape is maintained while the temperature is reduced below the Tg. The SMP can then be heated above the Tg and the original shape restored and subsequently maintained upon cooling below the Tg. Hence a polymeric system that can combine the properties of SMPs into SHPs would be advantageous for many devices and coatings that are employed where wounds to the polymeric material comprising the device or coating occur under normal use.